2. Introduction to oscilloscope
Architecture of CRT
Measurement of Oscilloscope
Lissajous Pattern (LP) methods
OUTLINE
3. The CRO (Cathode Ray Oscilloscope),
generally referred to as the oscilloscope or
simply “scope” is probably the most
versatile measuring instrument
Basically, CRO is a voltmeter with a screen.
Measurement of parameters:
◦ AC or DC voltage
◦ AC or DC current
◦ Time
◦ Phase relationship
◦ Frequency
INTRODUCTION TO OSCILLOSCOPE
4. Oscilloscope consists of the following parts:
◦ CRT – Cathode Ray Tube (Heart of Instrument)
◦ Vertical amplifier
◦ Horizontal amplifier
◦ Sweep generator
◦ Trigger circuit
◦ Associated power supplies
There are 2 types of oscilloscope: analog and
digital
However their principle and basic
characteristics are still the same
INTRODUCTION TO OSCILLOSCOPE …
6. Architecture of CRT
• A heated cathode to give off e .
electron gun
• An anode to accelerate e .
• The grid controls
brightness on the screen.
+
If grid is ve,
pushes back some e
fewer e reach screen
Filament
e beam
Vacuum
Electron gun
7. electron gun
• 2 sets of parallel plates
• (X-plates & Y-plates).
e beam is deflected by them
deflection system
+
filament
X-plates Y-plates
e beam
vacuum
Deflection system
8. electron gun deflection system
+
filament
X-plates Y-plates
fluorescent
screen
e beam
vacuum
• Coated with fluorescent material
Glows where e beam strikes it.
Fluorescent screen
light spot is formed.
spot
10. • With a d.c. voltage
the light spot is deflected towards +ve Y-plate.
11. • With a 50 Hz a.c. voltage,
the spot makes 50 oscillations per second.
appears as
a vertical line
12. • Voltages are applied to the Y-plate via a
built-in amplifier.
• The gain control adjusts the amplification of
the input voltage
to give a suitable deflection.
Vertical
Amplifier
Trigger
Generator
Horizontal
Amplifier
Horizontal plate
Vertical plate
Screen
Sweep
Generator
Input signal whose parameter is to be measured
13. Horizontal deflection
Time base circuit varies the voltage
across the X-plates.
The spot is made to sweep across the
screen at a steady speed, then, ‘fly
back’ rapidly to the start.
Amplitude
Time
Sudden fall
14. Waveform display
An a.c. voltage across Y-plates:
spot oscillates up & down
• Time base across X-plates:
spot sweeps across screen
+ =
15. MEASUREMENTS OF OSCILLOSCOPE
Voltage Measurements
Period and Frequency
Measurements
Phase Measurements or
Time Delay
16. The vertical scale is calibrated in either
volts per division or milivolts per division.
Using the scale setting of the scope and
the signal measured off the face of the
scope, it can measure peak-to-peak
voltage for an ac signal
VOLTAGE MEASUREMENT
Vp-p = (vertical p-p division) x (volts/div)
Vp-p = (no. of vertical division) x (volts/div)
OR
17. VOLTAGE MEASUREMENT …
(Volt/Div : 100mV/Div, Time/Div : 0.5ms/Div)
a) Voltage Peak-to-Peak
Vp-p= (V/Div) x No. of vert. div.
= 100 mV/div x (3.8 x 2)
= 0.76 V
b) Voltage Peak
Vp = (V/Div) x No. of vert. div.
= 100 mV/div x (3.8)
= 0.38 V
3.8
3.8
A B
Vp-p Vp
T
10
TD
2.5
18. PERIOD
◦ Horizontal scale of the scope can be used to
measure time in second, milisecond or
nanosecond.
◦ The interval of a pulse from start to end is
the period of the pulse.
FREQUENCY
◦ The measurement of a repetitive waveform
period can be used to calculate the signal
frequency.
PERIOD AND FREQUENCY
MEASUREMENT
Period = (horizontal p-p division) x (time/div)
F= 1/T
19. PERIOD AND FREQUENCY
MEASUREMENT …
3.8
3.8
A B
Vp-p Vp
T
10
TD
2.5
(Time/Div : 0.5ms/Div)
a) Period, T
T = (Time/Div) x (no. div/cycle)
= 0.5ms/div x 10
= 5ms
b) Frequency, f
f = 1/T
= 1/5ms
= 200 Hz
20. The time interval between waveforms is
called phase delay.
The phase delay is measured between the
midpoint at the start of each waveform
PHASE SHIFT (PHASE DIFFERENCE)
Phase difference,Ө = (phase difference in division) x (degrees/div)
21. PHASE SHIFT (PHASE DIFFERENCE)
3.8
3.8
A B
Vp-p Vp
T
10
TD
2.5
1 cycle = 10 div
TD = 2 div
Therefore,
1 cycle : 10 div = 360o
1 div = 360o / 10 = 36o
2 div = 2 x 36o = 72o
(Time/Div : 0.5ms/Div)
22. =0 = /4 = /2 =3 /4 =
=0 = /4 = /2 =3 /4 =
x-amplitude = y-amplitude
x-amplitude < y-amplitude
Apply 2 sinusoidal signals of SAME frequency to the X-plate (X-input)
and Y-plate (Y-input) of a CRO simultaneously.
23. FREQUENCY MEASUREMENT
◦ The alternative way of using oscilloscope to measure
frequency.
◦ A known reference frequency sine wave is applied to
one deflection plate and the unknown sinusoidal signal to
the other deflection plate
◦ A Lissajous pattern is produced on the screen according
to the frequency ration between the two signal.
LISSAJOUS PATTERN
fy max. no. of x-intercepts
fx max. no. of y-intercepts
=
24.
25. No. of x-intercepts: 3
No. of y-intercepts: 1
fy:fx = 3:1
No. of x-intercepts: 3
No. of y-intercepts: 2
fy:fx = 3:2
27. PHASE ANGLE MEASUREMENT
◦ Oscilloscope can be used in the X-Y mode to determine
the phase angle between two signals.
◦ This useful technique is limited to small frequency.
◦ The formula for phase angle measurement:
Where θ = phase angle in degree
Y1 = the distance from X-axis to the point where the Lissajous
pattern crosses Y-axis
Y2 = the maximum vertical distance on the Lissajous
X1 = the distance from Y-axis to the point where the Lissajous
pattern crosses X-axis
X2 = the maximum horizontal distance on the Lissajous
LISSAJOUS PATTERN …
Sin θ = Y1/Y2 = X1/X2
